A wavelength division multiplex is a high-speed and high-volume communication method that places a plurality of main optical signals of different wavelengths onto one optical fiber at the same time. As a technology that operates a high-speed and high-volume transmission network, a ROADM (reconfigurable optical add/drop multiplexer) is known that combines the wavelength division multiplex with a technology of a path management. The ROADM is a multiplexing system that adds and drops a reconfigurable main optical signal. In a ROADM system, ROADM nodes connected to a network are connected to one another in a ring using an optical fiber. Each ROADM node can drop a main optical signal of an arbitrary wavelength from a wavelength division multiplexed main optical signal and, conversely, it can add light of an arbitrary wavelength to a wavelength-division-multiplexed main optical signal. Accordingly, it is possible to extract a frame of a path without a conversion into an electrical signal, which permits a flexible path management while maintaining a high-speed transmission rate.
A ROADM node includes a WSS (wavelength selective switch) function that can output an input main optical signal into a different output port for each wavelength. When a WSS (wavelength selection) is performed in each ROADM node, PBN (passband narrowing) in which a transmission-path transmission band in a main optical signal becomes narrower occurs.
The transmission characteristics of light due to the occurrence of PBN vary according to a route (a path) in a transmission path. Further, the transmission characteristics of light in a multivendor network are also unknown, and there is also a possibility that the quality of a main optical signal will be deteriorated. If the transmission characteristics in a plurality of paths are measured in advance, it is possible to avoid, for example, the deterioration of the quality of a main optical signal. The transmission characteristics can be obtained from a power loss of light transmitted through the transmission path.
Regarding an optical multiplexing transmission system, a technology is known that evaluates a difference in wavelength between a transmission side and a reception side. Using a synchronization signal as a timing reference for a data acquisition, a processor extracts an intensity change component from an electrical signal obtained by a conversion performed by a receiver, so as to obtain a time waveform. On the basis of the time waveform, a waveform evaluator calculates a shift direction and a shift amount of an optical wavelength that are caused by a transmission performed from the transmission side to the reception side (see, for example, Patent Document 1).
A method for monitoring a transmission wavelength range of a wavelength tunable optical filter is known. A technology is known that monitors whether the transmission wavelength characteristics of a tunable wavelength filter of, for example, a WSS are properly controlled in an optical wavelength division multiplexing system. An optical signal light itself is frequency modulated, and an input/output power of an optical filter to be measured is measured through a variable bandpass filter, so as to obtain the transmission wavelength characteristics of a tunable wavelength filter from both monitoring results (see, for example, Patent Document 2).    Patent document 1: Japanese Laid-open Patent Publication No. 2004-364033    Patent document 2: Japanese Laid-open Patent Publication No. 2014-143614
When a transmission-path transmission band in a main optical signal is measured, for example, a transmission (alight source) side sweeps a center frequency of a CW (continuous wave) light in a frequency band to be measured. A reception side measures a power loss of the transmitted CW light for each center frequency of the CW light, so as to obtain the transmission characteristics of the transmission path.
Here, if the transmission side finely changes the center frequency and the reception side obtains the transmission characteristics accordingly, the transmission characteristics can be measured with a high degree of accuracy. On the other hand, if the center frequency is finely changed so as to obtain the transmission characteristics, there has been a problem in which a measurement time becomes longer.
Further, if the transmission side makes the extent of change in the center frequency larger and the reception side obtains the transmission characteristics every time the center frequency is changed, the measurement time can be made shorter. However, in this case, there has been a problem in which the accuracy of the transmission characteristics is lower even though the measurement time is made shorter.